IGZO display: Advantages and disadvantages

Commercial LCD panels incorporating IGZO display technology first appeared in 2012. The following years saw the emergence of televisions and other consumer electronic products such as smartphones and laptops featuring the same innovative display technology.

The iPad Mini 2 and iPad Air from Apple, as well as the Dell XPS 13 laptop from Dell Inc. and the Razer Blade 14 gaming laptop from Razer Inc. are some of the earlier products that featured IGZO display technology. With industry trend and market clamour gearing towards high definition and power-efficient display panels, IGZO is fast becoming a fitting display technology.

What is IGZO? How is IGZO used in display technology?

Professor Hideo Hosono, together with his team from Tokyo Institute of Technology and Japan Science and Technology Agency or JST, developed the crystalline IGZO-TFT in 2003 and amorphous IGZO-TFT in 2004.

JST owns the patent for IGZO-TFT and its applications. Samsung Electronics acquired a license in 2011 and Sharp Corporation followed suit in 2012. However, Sharp, in collaboration with Japan-based research company Semiconductor Energy Laboratories Co. Ltd., was the first to successfully manufacture IGZO-based display panels.

The “IGZO” name is an acronym for indium gallium zinc oxide. It is a semiconductor material made by crystalizing indium, gallium, zinc, and oxygen. Note that these elements are difficult to crystalize. However, successfully crystalizing them results in a new atomic arrangement and unique crystalline structure characterised by transparency and stability.

Transparency is a key characteristic of IGZO display. Note that every flat-panel display technologies consist of thin layers. These layers range from polarisers and light diffusers, to the actual LCD or OLED and electrodes. Transparency is critical to these layers. It allows light to pass through the entire panel and display the appropriate image.

A transistor also accompanies every pixel of the LCD or OLED layer so it can be turned on or off. A critical aspect of manufacturing flat-panel displays involves making these transistors transparent. In addition, the performance and size of the transistors also define the performance and quality of the flat-panel display.

Standard flat-panel displays features transistors fabricated from amorphous silicon or aSi. This semiconductor material is not transparent however. But manufacturers can etch them thin enough so that some light can pass through. Nonetheless, a naturally transparent IGZO-TFT means an ideal alternative to aSi-TFT.

In display technology application thereby, IGZO is a transparent thin-film transistors or TFT used in the backplane of flat-panel liquid crystal display or LCD or other panel technology. Most flat-panels today feature transistors made from amorphous silicon. The commercialisation of IGZO-TFT provides an alternative to silicon-based TFT.

What are the advantages of IGZO display technology?

The transparency of IGZO translates to a specific advantage. One advantage of IGZO-TFT is less power consumption. Compared with aSi-TFT LCD panels, IGZO-TFT panels use less power due to reduced need for backlighting intensity or illumination. The same is also true for aSi-TFT OLED panels. Because OLEDs can be made transparent, using IGZO-TFT reduces the brightness output from each organic diode.

Another critical advantage of IGZO is that it has 30 to 50 times the electron mobility of amorphous silicon. Higher electron mobility means there is less mass requirement for the same conductivity. When compare with aSi, electrons move faster in IGZO. In other words, the conductivity of IGZO transistors is just as good at a much smaller size. Remember that the performance and size of the transistors are critical to the performance and quality of flat-panel displays.

IGZO transistors also consume less power because of their small size. In displays with high pixel density, aSi-TFT would start to block a significant amount of backlight and obstruct the crammed and smaller-sized pixels. To compensate, aSi-TFT LCD displays consume more power for backlighting. Power consumption increases exponentially as the pixel density of aSi-TFT LCD displays increases. On the other hand, TFT LCDs based on the smaller IGZO transistors require lesser illumination.

Compared with aSi-TFT therefore, IGZO displays consume less power for backlighting while still producing brighter images. This technology is suitable for portable devices with high-resolution displays nonetheless.

The electron mobility in IGZO is also more efficient than aSi and even low-temperature polysilicon or LTPS. aSi has about a million to one ratio of on-current to off-current. However, it has relatively high leakage that results in high power consumption. LTPS is better than aSi because it has higher electron mobility. However, it also has higher leakage.

IGZO has electron mobility nearly as high as LTPS but with a significantly lower leakage current. Its on-current to of-current ratio is about a billion to one. This low leakage current combined with high electron mobility is another advantage of IGZO.

The advantageous current flow inside an IGZO display means that the transistors do not have to be continuously refreshed when a still image is on a screen. aSi and LTPS displays need to overwrite still image data through continued refresh or drive. Because the high leakage current in aSi or LTPS displays causes the pixels to discharge, power consumption is continuous. In IGZO displays however, the pixels remain charged because of low leakage current. IGZO displays essentially retain their active state longer than aSi or LTPS. This translates to lower power consumption, specifically in displaying still images such as documents or photographs. Accordingly, an IGZO display can save 80 to 90 percent power because it merely pauses the driving signals while maintaining a still image.

Another advantage of IGZO display is less noise influence. This comes from the fact that IGZO is an intermittent type of semiconductor. The drive in an IGZO display does not need to be continuous. In touch-sensitive displays, this means more sensitivity. While noise is a natural byproduct of any touch-sensitive displays, the noise generated by touching an IGZO-based touch panel is very brief. This enables a more accurate detection of even the faintest signal.

An ultra-sensitive IGZO display panel can pick up lines as thin as the tip of the pen. In application, this would mean that writing on an IGZO display has the same feel as on paper. The technology can enable the use of touch panels with natural handwriting input.

Conclusion: IGZO as a next generation display technology

In a nutshell, the advantages of IGZO display technology centres on its unique physical properties that enables the operation of power-efficient high resolution displays regardless of size. These advantages are beneficial in portable consumer electronics industry, particularly in manufacturing smartphones and tablet computers.

Remember that IGZO display technology is not a type of display panel but a type of TFT. The technology is applicable in other display technologies to include LCD TN and IPS, as well as OLED and AMOLED. Furthermore, there is no considerable difference between aSi-TFT LCD or OLED and IGZO-TFT display panels in terms of image quality and accuracy. However, IGZO allows substantial power saving, especially on portable devices with ultra-dense displays.

IGZO has some disadvantages nonetheless. The material is a metal oxide. The high reactivity of oxygen may result in very low voltage sensitivity. The uniformity of electron mobility in IGZO is lower than amorphous silicon or low-temperature polysilicon. In addition, the material uses the rare-earth metals indium and gallium. This rarity introduces risk in procurement. Cost might also be expensive.

Compared with aSi, manufacturing complication is another disadvantage of IGZO. The current roadblock to large-scale IGZO manufacturing is synthesis method. Pulsed Laser Deposition or PLD is the most common synthesis method in the industry. However, PLD requires expensive equipment and plenty of time for each sample to adjust to regular atmospheric conditions. The requirements can hamper mass production. Combustion synthesis is a viable alternative. However, there is a need for more research to evaluate its feasibility. These complications in manufacturing IGZO result in yield and cost trade-off.

The response time limitation of LCD technologies also creates disadvantage for IGZO. While IGZO can provide a faster response time than aSi or any silicon-based transistor, the slow response time of TN and IPS LCD technologies renders this apparent advantage negligible. OLED display technologies can take advantage of this faster response time however.